Microtetrahedronal Bi12TiO20/g-C3N4 composite with enhanced visible light photocatalytic activity toward gaseous formaldehyde degradation: Facet coupling effect and mechanism study

Article abstract of DOI:10.1016/j.molcata.2016.09.012

Uniform Bi₁₂TiO₂₀ microtetrahedrons exposed by {111} facets were synthesized by a controlled hydrothermal method. Well resolved density functional theory (DFT) calculations showed that {111} facet was the most active facet with the highest surface energy among the low index facets of Bi₁₂TiO₂₀. Series of facet coupled microtetrahedronal Bi₁₂TiO₂₀/g-C₃N₄ composites were prepared via a simple two-step method. The interface structure of composite was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), and fourier transform infrared (FT-IR) spectroscopy analyses, showing that the interface of Bi₁₂TiO₂₀/g-C₃N₄ composite was connected by the {111} facets of Bi₁₂TiO₂₀ and the {002} facets of g-C₃N₄. Owing to this typical structure, the Bi₁₂TiO₂₀/g-C₃N₄ composite with mass ratio 0.5:0.02 of Bi₁₂TiO₂₀ to g-C₃N₄ presented higher photocatalytic activity for degradation of gaseous formaldehyde under visible light irradiation (λ?≥?400?nm) as compared to the pure g-C₃N₄ and the as-obtained Bi₁₂TiO₂₀. Furthermore, the band structure calculations were employed to investigate the photocatalytic mechanism of Bi₁₂TiO₂₀/g-C₃N₄ composite.

Full text of DOI:10.1016/j.molcata.2016.09.012

Journal of Molecular Catalysis A: Chemical 424 (2016) 311–322
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Microtetrahedronal Bi TiO /g-C N composite with enhanced
12
20
3
4
visible light photocatalytic activity toward gaseous formaldehyde
degradation: Facet coupling effect and mechanism study
a
a
a,b,∗
b,∗∗
Hongli Sun , Jun Li , Gaoke Zhang
, Neng Li
a
School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070, PR China
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 430070, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2016
Received in revised form 22 August 2016
Accepted 7 September 2016
Available online 9 September 2016
Uniform Bi12TiO20 microtetrahedrons exposed by {111} facets were synthesized by a controlled
hydrothermal method. Well resolved density functional theory (DFT) calculations showed that {111}
facet was the most active facet with the highest surface energy among the low index facets of Bi12TiO20.
Series of facet coupled microtetrahedronal Bi12TiO20/g-C3N4 composites were prepared via a simple two-
step method. The interface structure of composite was characterized by scanning electron microscopy
(
SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), and fourier transform
Keywords:
infrared (FT-IR) spectroscopy analyses, showing that the interface of Bi12TiO20/g-C3N4 composite was
connected by the {111} facets of Bi12TiO20 and the {002} facets of g-C3N4. Owing to this typical structure,
the Bi12TiO20/g-C3N4 composite with mass ratio 0.5:0.02 of Bi12TiO20 to g-C3N4 presented higher photo-
catalytic activity for degradation of gaseous formaldehyde under visible light irradiation (␭ ≥ 400 nm) as
compared to the pure g-C3N4 and the as-obtained Bi12TiO20. Furthermore, the band structure calculations
were employed to investigate the photocatalytic mechanism of Bi12TiO20/g-C3N4 composite.
Bi12TiO20/g-C3N4 composite
Facet coupling structure
Formaldehyde degradation
DFT calculations
Visible light photocatalysis
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
about the relative magnitudes of its surface energy and its control
exposure of active facets.
Rencently, continuous efforts have been devoted to develop-
In addition, the construction of heterostructures via incorpo-
ration Bi-based materials with other semiconductors is also an
effective approach to promote the photocatalytic activity because
ing the efficient visible light driven photocatalysts due to the
increasing concerns of energy shortage and environmental pol-
lution [1–7]. Bismuth titanate Bi12TiO20, as a promising visible
light driven photocatalyst for degrading the organic pollutants,
has attracted wide attentions [8–13]. Yang et al. developed a
rapid microwave-assisted hydrothermal method for the synthesis
of the flowerlike Bi12TiO20 with hierarchical architecture, which
showed enhanced visible light photocatalytic activities towards the
degradation of Rhodamine B [14]. Zhu et al. sythesized Bi12TiO20
nanobelts and nanoparticles with improved photocatalytic per-
formance for degradation of acid orange 7 [15]. Furthermore, the
control synthesis of crystals with highly active facets has aroused
great research interest because of the unique facet-dependent
properties [16–26]. However, for Bi12TiO20, there were rare reports
the photogenerated charges can be efficiently separated. TiO [27],
2
PANI [28] and AgI [29] have been chosen to be the component of
some composites in order to obtain an increased photocatalytic
activity. To date, the graphitic carbon nitride (g-C N ) has been rec-
3
4
ognized as a promising candidate in fabricating the heterostructure,
as it offers plenty of active reaction sites and its two-dimensional
planar structure with ␲-conjugated system facilitates the charge
separation [30–34]. Particularly, some progresses have been made
in the research of facet coupling structure among g-C N -{002}
3
4
facets hybrid semiconductors. Ye et al. have studied the interaction
between g-C N -{002} facets and BiOBr-{001} facets and proved
3
4
that this facet coupling contributed to the high separation effi-
ciency of photo-induced charges [35]. Tang et al. reported that
the enhancement of visible light photocatalytic activity towards
the degradation of methyl blue was ascribed to the interrelation
between the active Ag PO -{111} facets and g-C N -{002} facets
∗ Corresponding author at: School of Resources and Environmental Engineering,
Wuhan University of Technology, Wuhan, 430070, PR China.
3
4
3
4
[
36]. Li et al. simultaneously investigated the relationships of g-
∗
∗
Corresponding author.
E-mail addresses: gkzhang@whut.edu.cn (G. Zhang), lineng@whut.edu.cn (N. Li).
C3N4 with BiOCl-{001} facets and BiOCl-{010} facets, and both
http://dx.doi.org/10.1016/j.molcata.2016.09.012
381-1169/© 2016 Elsevier B.V. All rights reserved.
1

3
12
H. Sun et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 311–322
exhibited significantly enhanced MO and phenol photodegradation
activity under visible-light irradiation [37].
(002)
g-C N4 (100)
( f )
3
Herein, surface energy calculations of Bi12TiO20 crystals were
carried out based on density function theory (DFT), and the {111}
0.5:0.08 Bi TiO20/g-C N4
12
3
( e )
( d )
( c )
facet of Bi TiO was proved to be the most active facet among
1
2
20
the {111}, {110} and {100} facets due to its highest surface
0
.5:0.04 Bi TiO20/g-C N4
1
2
3
energy. Based on this, Bi TiO with active {111} facets exposed
1
2
20
is expected to display excellent photocatalytic performance under
visible light irradiation. In this work, we successfully synthesized
the Bi12TiO20 tetrahedrons with nearly 100% {111} facets exposed
by using a simple hydrothermal method, and then developed a
0
.5:0.02 Bi TiO20/g-C N4
12
3
0.5:0.01 Bi TiO20/g-C N4
( b )
( a )
12
3
novel composite Bi12TiO20/g-C N4 via an ultrasonic method. The
3
(310)
(
321)
photocatalytic activity of the composite Bi12TiO20/g-C N was eval-
3
4
(
222)
uated by the degradation of gaseous formaldehyde under visible
light. Meanwhile, the facet coupling effect between Bi12TiO20
Bi12TiO20
(220)
-
{
111} facets and g-C N -{002} facets was studied.
3
4
5
10 15 20 25 30 35 40 45 50 55 60 65 70
2
. Experimental
.1. Synthesis of catalyst
All the chemicals were of analytical purity, and were used as
2 Theta(º)
Fig. 1. XRD patterns of (a) Bi12TiO20; (b) B/g-1; (c) B/g-2; (d) B/g-4; (e) B/g-8; and
f) g-C3N4.
2
(
received without further purification. The {111} facets exposed
were analyzed by a UV–vis spectrophotometer (UV2550, Shimadzu,
Japan), and fine BaSO4 powders were used as a reflectance standard.
Bi TiO microtetrahedrons were prepared by a hydrothermal
12
20
method. Typically, Bi O (12 mmol) and TiO2 (1 mmol) were dis-
2
3
solved in NaOH solution (4 mol/L, 60 mL) under magnetic stirring.
After that, the mixture was transferred to a stainless steel Teflon-
lined autoclave of 100 mL capacity. The autoclave was sealed and
2.3. Theoretical computational details
Surface energy and electronic structure were calculated with
a VASP package using plane wave basis sets based on the first
principle theory. Perdew-Burke-Ernzerh was used as the exchange-
correlation function. The surfaces were modeled by 2.5 thicknesses
without fixing any layers, and the vacuum separation between the
slabs was set to be 15 Å. The surface energy Esurf is defined as [38]:
◦
heated at 220 C for 0.5 h and then cooled to room temperature nat-
urally. The final products were collected by centrifugation, washed
◦
with distilled water and followed by drying at 70 C for 5 h.
Pure g-C N was prepared in a typical synthesis route. 10.0 g
3
4
melamine was put into an alumina crucible with a cover, heated
◦
◦
at 550 C for 4 h with a ramp rate of 2.5 C/min, and then cooled to
room temperature. The obtained yellow agglomerates were ground
into powder in a mortar.
Esurf = (Eslab − n × Ebulk)/(2 × A)
(1)
where Eslab is the total energy of the relaxed slab, Ebulk is the total
energy of the bulk, n is the number of the bulk unit cells in the
supercell to model the surface, and A is the surface area.
The Bi12TiO20/g-C N composites were fabricated by dispers-
3
4
ing 0.02 g of g-C N and 0.5 g of Bi12TiO20 microtetrahedrons in
3
4
the mixed solvent of ethanol and deionized water and then soni-
cating the mixture at 59 KHz for 6 h at ambient temperature. The
powders were collected by centrifugation, washed with deionized
2.4. Photocatalytic activity test
◦
water for three times, followed by drying at 70 C for 6 h. A series
The photocatalytic activity of the as-prepared samples was eval-
of Bi12TiO20/g-C N composites were synthesized by changing the
3
4
uated by the degradation of gaseous formaldehyde under visible
light irradiation. All experiments were carried out in a sealed,
cylindrical, and stainless gas-phase batch reactor with a volume
of 600 mL. A 300 W Dy lamp was used to irradiate the reactor,
and the visible light irradiation was achieved with a glass filter
amount of g-C N in the preparation procedure. The as-prepared
3
4
composites were designated as B/g-X, where X = 1, 2, 4, and 8,
denote the composites Bi12TiO20/g-C N4 with mass ratio 0.5:0.01,
3
0
.5:0.02, 0.5:0.04, and 0.5:0.08 of Bi12TiO20 to g-C N , respectively.
3 4
(
≥ 400 nm). The experiments were performed at room tempera-
ture as follows: 0.5 g of the as-prepared catalyst was dispersed
onto the surface of a dish with a diameter of 7 cm. After placing
the dish with catalyst in the reactor, 0.5 ␮L of formaldehyde was
injected with a syringe into the reactor. Once the concentration
of gaseous formaldehyde was stable, the lamp was switched on to
2.2. Characterization
The structure and crystallinity of the as-prepared samples were
characterized by X-ray diffraction (XRD) analysis on a D/MAX-RB
diffractometer with Cu K␣ radiation under the operation conditions
of 40 kV and 50 mA. SEM (JSM-5610LV) was used to characterize
the morphology. Transmission electron microscopy, high resolu-
tion transmission electron microscopy (HRTEM) and selected area
electron diffraction (SAED, JEM2100F, 200 kV) were taken to char-
acterize the morphology and microstructure of the products. X-ray
photoelectron spectroscopy (VG Multilab2000) was employed to
analyze the valence states of the elements with a monochro-
matic Mg K␣ source and a charge neutralizer. The binding energies
obtained in the XPS spectral analysis were corrected for specimen
charging by referencing C 1 s to 284.5 eV. A Nexus Fourier transform
infrared spectroscopy (Thermo Nicolet, USA) was used to detect the
chemical bonds of the samples. The absorption edges of the samples
initiate the photocatalytic reactions. The concentrations of CO and
2
gaseous formaldehyde were constantly measured by a photoacous-
tic field gasmonitor (Innova1412, AirTech Instruments, Denmark)
every 15 min interval.
3. Results and discussion
3.1. Structure and crystallinity
The XRD patterns of the samples g-C N , Bi12TiO20 and
3
4
Bi12TiO20/g-C N4 prepared under different conditions are shown
3
◦
◦
◦
◦
in Fig. 1. The diffraction peaks at 2ꢀ = 24.60 , 27.58 , 30.28 , 32.74 ,

H. Sun et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 311–322
313
Fig. 2. SEM images of (a) Bi12TiO20; and (b) B/g-2; (c) SAED image of Bi12TiO20; (d) HRTEM image of B/g-2.
◦
◦
◦
◦
◦
◦
4
1.40 , 43.32 , 45.20 , 52.14 , 53.80 , and 55.40 in Fig. 1a-e
cates that the selected single crystal is projected against a plane
-
can be assigned to (220), (310), (222), (321), (332), (422), (510),
530), (600) and (611) of the cubic phase Bi12TiO20 with space
group I23 (JCPDS 34-0097), respectively. And the peak at 27.7 in
Fig. 1f is indexed as the (002) diffraction plane of the tetragonal
phase g-C N (JCPDS 87-1526). The strong and sharp diffraction
peaks indicate that the as-prepared samples have good crystallinity
and no other impurity peaks are detected. For the composite
Bi12TiO20/g-C N , the characteristic diffraction peaks of the g-C N
are not observed, which is mainly due to the low content level of g-
C N . Besides, the peak of the (002) diffraction plane is close to that
perpendicular to the [088] zone axis, confirming that the {111}
(
facets are exposed on the surfaces of the Bi12TiO tetrahedrons
20
◦
[
39–41].
As shown in Fig. 2b, the composite B/g-2 has a relatively rough
3
4
surface with some slight ditches and dot-like defects as compared
to the pure Bi12TiO20 with a smooth surface. It indicates that the
g-C N was coated on the exposed facet of Bi12TiO20, forming a
3
4
3
4
3
4
rough face with some steps, ledges and kinks. HRTEM image in
Fig. 2d shows the clear lattice fringes in B/g-2. The spacings of
0.5077 nm and 0.7192 nm are indexed to the facets {200} and {110}
of Bi TiO , respectively. And the spacing of 0.3253 nm corre-
3
4
of the (310) diffraction plane of Bi12TiO20, making the diffraction
peaks of the g-C N can not be easily distinguished.
12
20
3
4
sponds to the {002} facets of g-C N . HRTEM demonstrates that
3
4
g-C N covers the {111} facets of Bi TiO , and the facet coupling
3
4
12
20
3
.2. Morphology and microstructure
of the composite Bi12TiO /g-C N happened among Bi12TiO20-
20 3 4
{
111} facets and g-C N -{002} facets.
3
4
Fig. 2 shows the morphology and microstructure of the pure
Bi12TiO20 and the composite B/g-2. As indicated in Fig. 2 a and S2,
most of the as-synthesized Bi12TiO20 are 3D tetrahedronal crys-
tals with a uniform size of about 3 ␮m. The selected area electron
diffraction (Fig. 2c) confirms that the exposed facets of tetrahe-
3.3. XPS analysis
The chemical compositions and elemental states of the com-
dronal Bi1 TiO20 are {111} facets. The reciprocal lattice spacings of
posite Bi12TiO20/g-C N4 were further studied by XPS technique. It
2
3
−
1
−1
2
.000 nm and 6.848 nm in the SAED correspond to the fringe
can be seen from Fig. 3a that the obtained composite B/g-2 is com-
posed of Bi, O, Ti, C and N elements. As shown in Fig. 3b, the peaks
of Ti 2p1/2 located at 463.5 eV and the peak of Ti 2p3/2 located at
457.2 eV correspond to the binding energies of the level of Ti (IV)
[42,43]. The O 1 s binding energy peak at around 529.2 eV (Fig. 3c)
spacings of 4.999 Å and 1.460 Å, which could be indexed to the (200)
and (444) reflections of body-centered cubic Bi12TiO20, respec-
◦
tively. The angle between (200) and (444) facets is about 54.79 ,
◦
which is in agreement with the theoretical angles of 54.73 . It indi-

3
14
H. Sun et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 311–322
500000
400000
300000
200000
100000
0
Bi 4d
(
a )
0.5:0.02 Bi TiO /g-C N
3/2
12
20
3
4
( b )
5
4
4
3
0000
5000
0000
5000
465.6eV
Ti 2p
463.5eV
1
/2
Ti 2p3/2
57.2eV
4
30000
0
200
400
600
800
1000
1200
470
465
460
455
Binding Energy (eV)
Binding Energy (eV)
6
5
5
4
4
0000
5000
0000
5000
0000
80000
(
c )
O1s
(
d)
Bi 4f
5
29.2eV
Bi 4f
158.3eV
7/2
70000
60000
50000
Bi 4f
163.8eV
/2
5
5
26.7eV
40000
30000
20000
10000
5
34
532
530
528
526
524
166
164
162
160
158
156
154
Binding Energy (eV)
Binding Energy (eV)
2
2
2
2
2
2
2
2
9000 ( e )
30000 ( f )
N 1s
C 1s
284.6eV
8000
29000
28000
27000
26000
25000
24000
286.5eV
397.5eV
398.6eV
7000
396.3eV
6000
399.6eV
2
88.1eV
5000
4000
3000
2000
290
288
286
284
282
402
400
398
396
394
Binding Energy (eV)
Binding Energy (eV)
Fig. 3. High-resolution XPS spectra of (a) full elements; (b) Ti 2p; (c) O 1s; (d) Bi 4f; (e) C 1s; (f) N 1s.
is ascribed to O atoms bound to metals (Bi
O
Bi bonds and Ti
O
titious carbon species [52], and the peak at 286.5 eV is assigned to
C carbon [53]. For the peak at 288.1 eV, it can be ascribed to
(N)2 carbon [35,52], or the Ti C structure [46], which
agrees well with the existence of the rare peak of the O 1 s at
526.7 eV. The N 1 s spectrum in Fig. 3f is divided into four peaks
(399.6 eV, 398.6 eV, 397.5 eV and 396.3 eV) after Gaussian curve fit-
bonds) [44–46]. And as compared to the O 1 s spectrum of pure
Bi12TiO20 (Fig. S3a), the peak of the O 1 s spectrum with a lower
binding energy (526.7 eV) in the composite is assigned to the charg-
C N
the N
C
O
ing effect bound to g-C N4 with particular conductivity properties
3
[
47]. The binding energies of Bi 4f
and Bi 4f5/2 in Fig. 3d reflect the
7/2
3
+
characteristics of Bi [27,48–51]. The C 1 s spectrum in Fig. 3e rep-
resents mainly three peaks with the binding energy at 284.6, 286.5
and 288.1 eV. The peak located at 284.6 eV results from the adven-
ting. The signals with the binding energy at 399.6 eV and 398.3 eV
2
show the occurrence of sp -hydridized nitrogen C
N C and ter-
tiary pyrrolic nitrogen N–(C)3, respectively [54–57]. As compared

H. Sun et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 311–322
315
Fig. 4. (a) Side view and (b) top view of proposed model of the composite Bi12TiO20/g-C3N4. The red, white, purple, blue and grey spheres represent O, Ti, Bi, N and C atoms,
respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
intensity of the band at 805 cm 1 slowly increased with increas-
−
(
(
f )
ing the content of g-C N4 in the composite. The result indicates
that the g-C N4 is indeed coated on the surface of the tetrahe-
dronal Bi12TiO20 and confirms the formation of the facet coupling
0
.5:0.08 Bi TiO
20
/
g-C N
3
1
2
3
4
3
e )
0
0
0
.5:0.04 Bi TiO /g-C N
1
2
20 3
4
( d )
( c )
( b )
among g-C N4 and the {111} facets exposed Bi12TiO20. The FT-IR
3
analysis result is in consistent with those of XRD, TEM and XPS
analyses.
.5:0.02 Bi TiO20/g-C N
12
3
4
4
.5:0.01 Bi TiO20/g-C N
12
3
3.5. Optical absorption property and band structure calculation
8
27
Bi TiO
20
12
Fig. 6a shows the light absorption property of the as-obtained
6
65
5
90 462
g-C N
samples. The light absorption edge of the pure Bi12TiO20 and the
pure g-C3N4 respectively occurs at approximately 488 nm and
460 nm, indicating that they could absorb considerable amounts of
visible light and have potential visible light photocatalytic activity.
3
4
525
(
a )
1316
1
410
1240
805
800
1640
The band gap energy of the pure Bi12TiO20 and the pure g-C N4
could be estimated to be about 2.5 eV and 2.7 eV respectively,
according to the formula [28,65]:
3
2
000 1800 1600 1400 1200 1000
600
400
-1
Wavenumbers (cm )
n/2
˛
hꢁ= A(hꢁ-Eg)
(2)
Fig. 5. FT-IR spectra of (a) g-C3N4; (b) Bi12TiO20; (c) B/g-1; (d) B/g-2; (e) B/g-4; and
f) B/g-8.
(
where ␣, ꢁ, Eg and A are absorption coefficient, light frequency,
band gap, and a constant, respectively. And the absorption spec-
tra of the composites Bi12TiO20/g-C N4 show similar features as
compared to that of pure Bi12TiO20, implying that the introduction
3
to the N 1 s binding energy spectrum of pure g-C N , the appear-
3
4
ance of the two new peaks at 397.5 eV and 396.3 eV may be arisen
from the Ti N bond in terms of partial substitutional doping of
N for O sites [58–61]. Fig. 4 illustrates the proposed model of the
of a small quantity of g-C N4 wouldn’t affect the light absorption
ability. Moreover, in the region of wavelength below 430 nm, the
composites show some slight shoulders which is ascribed to the
3
composite Bi12TiO20/g-C N4 according to the XPS results, display-
coating of g-C N4 on the {111} facets of the Bi12TiO20. The result
3
3
ing that the facet coupling effect between Bi12TiO20 {111} facets
agrees with those of XRD, TEM and FT-IR analyses.
and g-C N {002} facets gets enhanced by the strong interaction
Fig. 6b displays the calculated band structure of tetrahedronal
Bi12TiO20, illustrating that the {111} facets exposed Bi12TiO20
shows an indirect band gap with a value of 1.45 eV. As compared
to the experimental value (2.5 eV), it is underestimated because
of the well-known limitations of exchange-correction function in
describing excited states in the DFT calculations [66].
3
4
between elements Ti and N as well as elements C and O.
3.4. FT-IR analysis
FT-IR spectra of Bi12TiO20
, g-C N and the composite
3 4
Besides, the band positions of the as-synthesized g-C N4 and
3
Bi12TiO20/g-C N are depicted in Fig. 5. The bands at 462, 525, 590
3
4
Bi12TiO20 were estimated by using the empirical equations [67]:
−1
and 665 cm in Fig. 5b-f are ascribed to the characteristic Bi
O
vibration modes of sillenite Bi12TiO20 [13,48,62]. Meanwhile, the
EVB = X − Ee + 0.5Eg0·5Eg
ECB = EVB − Eg
(3)
(4)
−1
band at 827 cm is due to the telescopic vibration absorption of
−
1
Bi
O
Bi [63]. The band at 805 cm
in Fig. 5a, c-f corresponds
to the typical breathing mode of the triazine ring of g-C N [56].
where EVB and ECB are the valence band and conduction band edge
potentials, respectively, X is the electronegativity of the semicon-
ductor, which is the geometric mean of the electronegativity of
the constituent atoms, Ee is the energy of the free electrons on
the hydrogen scale (approximately 4.5 eV), and Eg is the band gap
energy of the semiconductor. The calculated results are illustrated
3
4
−
1
And the strong bands within 1240–1640 cm , with the dominat-
−
1
ing characteristic bands at 1251, 1316, 1410 and 1640 cm , are
associated with the typical stretching and rotation vibration of the
C
N and C N bonds in heterocycles [34,64]. It can be noted that the
−1
band at 827 cm in the composite gradually disappeared and the

3
16
H. Sun et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 311–322
(
a )
1.0
0.8
0.6
0.4
0.2
Bi TiO
20
1
2
g-C
N
3
4
0.5:0.01 Bi TiO /g-C
N
1
2
20
3
4
4
4
4
0
.5:0.02 Bi TiO /g-C N
1
2
20 3
0
.5:0.04 Bi TiO /g-C N
1
2
20 3
0
.5:0.08 Bi TiO /g-C
N
1
2
20
3
Photolysis
0
1
2
3
4
5
6
7
Time (h)
8
7
7
6
00
50
00
50
(
b )
1
1
1
1
60
40
20
00
HCHO
CO
2
8
6
4
2
0
0
0
0
600
550
0
1
2
3
4
5
6
7
Time (h)
st
( c )
1
.0
.9
.8
.7
.6
.5
1
2
3
4
5
nd
th
th
th
0
0
0
0
0
Fig. 6. (a) UV–vis DRS spectra of Bi12TiO20, g-C3N4 and the Bi12TiO20/g-C3N4 com-
posites; (b) Band structure of Bi12TiO20; (c) Band positions of Bi12TiO20 and g-C3N4.
in Fig. 6c, and a staggered band structure for single g-C N4 and
3
Bi12TiO20 is observed, which seems to be favorable for the separa-
tion of photogenerated carriers.
0.4
0
.3
.2
3.6. Enhanced photocatalytic activity
0
0
1
2
3
4
5
6
7
We firstly evaluated the photocatalytic activity of the irregu-
lar shaped Bi12TiO20 (IR-BTO) and tetrahedronal Bi12TiO20 (T-BTO).
Their photocatalytic activity toward gaseous formaldehyde degra-
dation were shown in Fig. S1. It can be seen that the Bi12TiO20
tetrahedrons exposed with {111} facets displayed higher photo-
catalytic activity than the irregular shaped Bi TiO , indicating
Time (h)
Fig. 7. (a) Photocatalytic degradation of HCHO by Bi12TiO20, g-C3N4 and the
Bi12TiO20/g-C3N4 composites; (b) The changes of CO2 concentration during HCHO
photocatalytic degradation by the B/g-2; (c) cycling runs of the B/g-2 for photocat-
alytic degradation of HCHO.
12
20
that there is a certain correlation between surface energy and
photocatalytic activity. Then the photocatalytic activity of the
Bi12TiO20/g-C N composites was evaluated by the degradation of
3
4
gaseous formaldehyde HCHO. Fig. 7a shows that all the compos-

H. Sun et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 311–322
317
Fig. 8. Top views of Bi12TiO20 model exposed with single (a) {111} facet; (b) {100} facet; and (c) {110} facet; Side views of Bi12TiO20 model exposed with single (d) {111}
facet; (e) {100} facet; and (f) {110} facet. The red, white and purple spheres represent O, Ti and Bi atoms, respectively. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
Table 1
Calculated clean surface energies of different facets of Bi12TiO20.
Facet
bulk
Thickness
Relaxed energy
a
b
Surface area (Å2)
Surface energy (J/m2)
/
−395.707
−987.878
−1031.444
−978.029
/
/
/
/
100
110
111
2.5
2.5
2.5
10.2173
10.2173
14.4494
10.2173
14.4494
12.5135
104.3932
147.6339
180.8133
0.106
−2.285
0.497

3
18
H. Sun et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 311–322
a) 800
140
120
100
80
(
(
b) 200
{
111} facets exposed Bi TiO
12 20
700
600
500
400
300
200
100
0
180
Total
Bi
O
Ti
6
0
160
140
40
20
0
-5
-4
-3
-2
-1
0
1
2
3
4
5
1
20 Element Bi
s
100
p
d
80
60
40
20
0
-
20
-15
-10
-5
0
5
-20
-15
-10
Energy / eV
-5
0
5
Energy / eV
(
c) 1400
1000
00
600
90
60
50
40
(d)
8
8
0
0
1200
3
0
4
2
0
00
00
7
20
10
0
1000
60
-
5
-4
-3
-2
-1
0
1
2
3
4
5
-5
-4
-3
-2
-1
0
1
2
3
4
5
8
6
4
2
00 Element O
5
0
Element Ti
s
s
p
d
40
00
p
d
30
20
10
0
00
00
0
-
20
-15
-10
-5
0
5
-20
-15
-10
-5
0
5
Energy / eV
Energy / eV
Fig. 9. (a) Total density of states for {111} facets-exposed Bi12TiO20; Partial density of states of the element (b) Bi; (c) O; and (d) Ti.
ites showed higher activity than both the pure Bi12TiO20 and the
pure g-C N for the photocatalytic degradation of the HCHO, indi-
high photocatalytic efficiency after five cycles, suggesting that the
synergistic facet coupling effect between Bi12TiO20 and g-C N4 is
3
4
3
cating that a synergy effect exists between Bi12TiO20 and g-C N .
stable.
3
4
Furthermore, we also evaluated the photocatalytic activity of the
compound (PM-B/g-2), which is only the simple physical mixture
of Bi12TiO20 and g-C N without any treatment, as comparison to
3
4
confirm the synergistic effect. The results were shown in Fig. S4. As
shown in Fig. S4, the composite B/g-2 displayed enhanced photo-
catalytic activity toward gaseous HCHO, better than pure Bi12TiO20
3.7. Surface energy of Bi12TiO20
tetrahedrons, pure g-C N4 and the simply physically mixed com-
Fig. 8 illustrates the surface models for {111}, {100} and {110}
3
posite PM-B/g-2. These results further proved that there is a synergy
effect within the composite Bi12TiO20/g-C N . The photocatalytic
facets exposed Bi12TiO . For each facet, the surface model is con-
20
3
4
structed on the basis of a slab model with same vacuum regions of
15 Å. Based on the well resolved DFT calculations, the clean surface
energies of each facet of Bi12TiO20 are summarized in Table 1. It is
worth noting that the theoretically highest energy facet is found to
activity of the Bi12TiO20/g-C N composites first increased and then
3
4
decreased with increasing the g-C N content in the composite. The
3
4
composite B/g-2 showed the highest photocatalytic efficiency. The
decrease of the photocatalytic activity of composites with higher
content of g-C N could be ascribed to the overloading of g-C N .
2
2
be {111} facet (∼0.497 J/m ) rather than {100} facet (∼0.106 J/m )
2
3
4
3
4
or {110} facet (∼−2.285 J/m ), indicating that the {111} facet of
As the film coated on the Bi12TiO20 {111} facets might too thick to
separate the charges efficiently. And it might act as the recombi-
nation centers of photo-induced electron-hole pairs and partially
suppress the effective facet coupling effect among Bi12TiO20-{111}
facets and g-C N -{002} facets. As shown in Fig. 7b, the concentra-
Bi TiO is the most active facet among the low index facets. And
12
20
the successful synthesis of the Bi12TiO20 tetrahedrons with nearly
100% {111} facets exposed offer a perfect base to be coated by g-
C N nanosheet with layer structure, forming a pure facet coupling
3
4
3
4
between {111} facets and {002} facets. Generally, crystals prefer to
expose thermodynamically stable facets during the crystal growth
process as a result of the minimization of surface energy. However,
the presence of morphology controlling agents would certainly
change the preferable facet exposure [68,69]. The preferable {111}
facet exposure of this experiment might be attributable to the effect
tion of CO increased by around 200 ppm while HCHO decreased
2
by 130 ppm during the photocatalytic process, indicating that
the reduced HCHO was almost oxidized into CO . The observed
2
increase in the CO2 concentration is larger than the decrease in
the HCHO concentration, mainly due to desorption of some HCHO
molecules from the reactor surface during experiment and their
−
of OH anions on specific surfaces in the reaction process, which
subsequent oxidation to CO , as well as the measurement accu-
influenced the direction and rate of crystal growth, and eventually
resulted in the exposure of the desirable facets. [38,70–72].
2
racy of CO . Fig. 7c show that the composite B/g-2 still exhibited
2

H. Sun et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 311–322
319
Fig. 10. A schematic illustration of facet coupled effect and electron-hole pairs separation of the composite (a) before contact, and (b) after contact.
3
.8. Density of states of {111} facet exposed Bi12TiO20
sity of states (PDOS) of the {111} facet exposed Bi12TiO20 were
calculated based on the well resolved ab initio calculations. As
shown in Fig. 9, the energy range of the DOS from 20 eV to 5 eV
started with mainly O 2 s states (−19 to −16 eV), Bi 6 s states (−11
To further investigate the role of tetrahedronal Bi12TiO20 in the
composites, the total density of states (TDOS) and the partial den-

3
20
H. Sun et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 311–322
to −7.5 eV), O 2p and Bi 6p (−5.5 to 0 eV), and then mixing of Bi 6p, O
p and Ti 3d (1to 4.5 eV). And it is found that the valence band max-
well-contacted g-C N -{002} facets and Bi12TiO20-{111} facets.
3
4
2
The work can provide a new insight into designing high effi-
cient Bi12TiO20-based photocatalysts by controlling exposed crystal
facet.
imum (VBM) and the conduction band minimum (CBM) of {111}
facet-exposed Bi TiO are mainly dominated by O 2p states and Bi
1
2
20
6
p states, respectively, which agrees well with the previous report
[
73]. Moreover, both the VBM and CBM are consisted of Bi 6p and
Acknowledgements
O 2p orbitals, implying the existence of some covalent bond char-
acter between Bi and O atoms [74]. And the interaction between Bi
and O atoms might favor the generation and separation of the pho-
toexcited electron-hole pairs and thus enhance the photocatalytic
activity [65].
This work was supported by National Natural Science Founda-
tion of China (51472194), the National Basic Research Program of
China (973 Program) 2013CB632402 and the Natural Science Foun-
dation (NSF) of Hubei Province (No.2015CFB227).
Appendix A. Supplementary data
3.9. Mechanism of enhanced photocatalytic activity of the
composite
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.09.
It is well known that the separation efficiency of photo-
0
12.
generated electrons and holes plays an important role for the
enhancement of photocatalytic activity [75–78]. Fig. 10 schemat-
ically illustrates the transferring mechanism of photo-generated
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